Highly efficient catalytic degradation of organic dyes using iron nanoparticles synthesized with Vernonia Amygdalina leaf extract

Today, nanoscience explores the potential of nanoparticles due to their extraordinary properties compared to bulk materials. The synthesis of metal nanoparticles using plant extracts is a very promising method for environmental remediation, which gets global attention due to pollution-led global warming. In the present study, iron nanoparticles (FeNPs) were successfully synthesized by the green method using Vernonia amygdalina plant leaf extract as a natural reducing and capping agent. Biosynthesized FeNPs were characterized with different analytical techniques such as UV–visible, FT-IR, XRD, and SEM. The analysis revealed the formation of amorphous FeNPs with an irregular morphology and non-uniform distribution in size and shape. The average particle size was approximately 2.31 µm. According to the catalytic degradation investigation, the FeNPs produced via the green approach are highly effective in breaking down both CV and MB into non-toxic products, with a maximum degradation efficiency of 97.47% and 94.22%, respectively, when the right conditions are met. The kinetics study exhibited a high correlation coefficient close to unity (0.999) and (0.995) for the degradation of MB and CV, respectively, for the zero-order pseudo-kinetics model, which describes the model as highly suitable for the degradation of both dyes by FeNPs compared to other models. The reusability and stability of biosynthesized nano-catalysts were studied and successfully used as efficient catalysts with a slight decrease in the degradation rate more than four times. The results from this study illustrate that green synthesized FeNPs offer a cost-effective, environmentally friendly, and efficient means for the catalytic degradation of organic dyes.

www.nature.com/scientificreports/metal oxide nanoparticles, need to be prepared.Various methods, such as physical, chemical, and biological methods, are used for the synthesis of nanoparticles.The biological method (green method) is the preferred technique because it is more cost-effective, eco-friendly, and easy to use than the other methods 39 .Moreover, plant extracts naturally act as reducing, stabilizing, and capping agents for the synthesis of NPs.Iron nanoparticles (FeNPs) are considered efficient catalysts for dye degradation in the presence of BH 4  -as an ion donor due to their large surface area, high reactivity, and outstanding photochemical stability 33 .The specific surface area of FeNPs is 33.5 m 2 g −1 , while that of granular iron is only 0.9 m 2 g −1 , which is much less than that of nanosized iron 40 .
Vernonia amygdalina is a tree that belongs to the Asteraceae family and is a very common plant that grows predominantly in the eastern and western parts of tropical Africa.In addition, it is popularly called bitter leaf in English.Vernonia amygdalina grows in most parts of Ethiopia and is known as the 'grawa' in Amharic 16 .The leaves of Vernonia amygdalina have been found to be relevant in traditional folk medicine as an anthelming agent, a laxative herb, and an antimalarial agent 39 , as they are known as quinine substitutes 40,41 .
Vernonia amygdalina is a plant that has been traditionally used for its antibacterial properties and for maintaining the health of organs such as the kidney and liver, thanks to its therapeutic elements such as saponins, venomygdin, and vernodalin 38 .Leaf decoctions are used to treat fever, malaria, diarrhea, dysentery, hepatitis, and cough; as laxatives; and as fertility inducers 16 .They are also used as medicines for scabies, headaches, and stomachaches.Root extracts are also used as treatments for malaria and gastrointestinal disorders 41 .However, despite its excellent medicinal properties, its pharmacological and environmental actions have not been fully explored.Our research aimed to address this gap by using Vernonia amygdalina extracts as stabilizers to control the size of FeNPs during synthesis.We hope that this will promote the use of Vernonia amygdalina in environmental applications.Our study describes the environmentally friendly biosynthesis and characterization of iron nanoparticles using Vernonia amygdalina extracts as reducing and capping agents.Additionally, we report the characterization and application of these nanoparticles in the catalytic degradation of crystal violet and methylene blue.

Visual inspection for the synthesis of iron nanoparticles
NPs exhibit a different array of colors during synthesis.Plant extracts contain several phytochemicals that react with metal ions and convert them to nanoparticles 42 .During the synthesis of the iron nanoparticles, a change in color from light brown to grayish black was observed within 5 min after the addition of the Vernonia amygdalina leaf extract, indicating the formation of iron nanoparticles (Fig. 1).After 24 h of reaction, the color change stopped, and precipitation was observed, confirming that the nanoparticle synthesis process was complete 43 .

Ultraviolet-visible spectroscopy analysis of FeNPs
The successful formation of the biosynthesized FeNPs was confirmed by measuring their absorbance at wavelengths ranging from 200 TO 800 nm via UV-vis spectroscopy.This technique is a powerful tool for detecting the formation of nanoparticles on the surface plasmon resonance (SPR) peak and for calculating their band gap energy.The continuous absorption of the UV-Vis spectrum in the SPR region confirmed the formation of greenly synthesized FeNPs, which were observed to be nanoscale particles (as depicted in Fig. 2).This observation suggested that the biosynthesized FeNPs may be amorphous and polydispersed.Additionally, the absorbance spectrum of FeNPs obtained using aqueous Vernonia amygdalina leaf extract was similar to that of previous reports on FeNPs synthesized using aqueous tea and sorghum extracts 44 , Catharanthus roseus leaf extracts 45 , and Calotropis gigantea flower extract 46 .

Fourier transform infrared (FT-IR) spectral analysis of the FeNPs
Fourier transform infrared (FT-IR) spectroscopy was employed in this study to determine the surface composition of the synthesized FeNPs, mainly the specific functional groups that may contribute to the formation of FeNPs.Thus, this technique provides information on the interactions between phytochemicals present in plant extracts and the metal ions responsible for the production and capping of iron nanoparticles.This was confirmed X-ray diffraction (XRD) analysis of the FeNPs X-ray diffraction (XRD) analysis was employed to explore the surface crystallinity of the biosynthesized iron nanoparticles, revealing predominantly amorphous FeNPs supported by indistinct diffraction peaks (Fig. 4).
Similarly, prior investigations have reported the production of amorphous FeNPs using leaf extracts from diverse  plant sources, such as eucalyptus, pomegranate, and cherry 48 .The presence of organic components, derived from the leaf extract, is indicated by a broad shoulder peak observed within the 15° to 30° range of 2θ values, suggesting their role as capping and stabilizing agents for the synthesized iron nanoparticles 49 .The XRD spectrum of FeNPs displays broadening diffraction and a lack of well-defined peaks, signifying the polydispersion of the nanoparticles, and the presence of multiple peaks at different angles suggests a variation in size.This pattern aligns well with the findings from the SEM images and PSA analysis.Notably, analogous FeNP patterns were observed in studies harnessing eucalyptus, pomegranate, and cherry aqueous extracts 50,51 .

SEM and PSA for biosynthesized FeNPs
The morphology, microstructure, and particle size distribution of the synthesized FeNPs were assessed using scanning electron microscopy (SEM) images and particle size analysis (PSA).SEM images of FeNPs synthesized with Vernonia amygdalina leaf extract at various magnifications are shown in Fig. 5a and b, revealing the successful synthesis of FeNPs.According to these images, the formed nanoparticles exhibit an irregular morphology and lack a uniform distribution in shape and size.By utilizing Image J software through SEM, the average particle size distribution of the FeNPs was estimated to be approximately 2.31 µm, as displayed in Fig. 5c and d.The analysis indicated that approximately 90% of the particles fell within the 1-4 µm range, while the remaining particles were outside this range.This result is consistent with that of a previous study on the biosynthesis of FeNPs from loquat Eriobotrya japonica leaves, which reported an average particle diameter of 114.3 nm and 0.114.3µm 52.The biosynthesized FeNPs from the leaf extracts of Vernonia amygdalina were found to be polydispersed based on the XRD and UV-Vis spectra, with a majority of them exhibiting a variable size with a smaller diameter.The results were confirmed by identifying size variation, with an average diameter of approximately 2.31 µm for the biosynthesized FeNPs, suggesting their relatively smaller size.

Control experiment
Sodium borohydride is known to facilitate the reduction of crystal violet and methylene blue.However, when used in the absence of FeNPs, this process occurs slowly.As depicted in Fig. 6, after 60 min, only 25.10% and 15.73% of the crystal violet and methylene blue, respectively, were reduced.This is due to the large difference in redox potential between the donor and the acceptor, which makes the reduction thermodynamically favorable but not kinetically favorable.The addition of FeNPs as a nanocatalyst provides an alternative route with low activation energy for the reduction reaction, reducing the kinetic barrier and making the reaction kinetically favorable.Furthermore, the FeNPs provide a suitable surface for binding crystal violet and methylene blue particles and borohydride ions (BH 4 −1 ) to interact with each other, leading to the formation of decomposition products.These nanoparticles make the degradation of crystal violet and methylene blue kinetically feasible, resulting in complete reduction within a short period of time 53,54 .

Effect of catalyst loading
To determine the optimal quantity of catalyst required for the efficient degradation of crystal violet and methylene blue, various concentrations of aqueous solutions containing the dyes at a concentration of 10 ppm were tested in the presence of varying amounts of the FeNP catalyst.The results, which are presented in Fig. 7 and 'Tables S1 and S2' , indicated that the degradation efficiency increased as the amount of FeNP catalyst increased from 0.010 to 0.025 gm for crystal violet and from 0.010 to 0.075 gm for methylene blue.This can be attributed to the greater availability of surface active sites for BH 4 −1 ion and dye molecule adsorption with increasing FeNP dose 37 .Based on these findings, optimum catalyst loads of 0.025 gm for crystal violet and 0.075 g for methylene blue were identified, resulting in degradation percentages of 95.7% and 93.15%, respectively.It is worth noting www.nature.com/scientificreports/ that beyond the optimum catalyst load, the efficiency decreases due to the aggregation of nanoparticles at high catalyst doses, which may contribute to a lower degradation percentage by reducing the number of active sites 55 and decreasing the electron density.

Initial concentration of dyes
The results of the present study, which involved examining six different concentrations of aqueous solutions containing crystal violet and methylene blue ranging from 5 to 30 ppm, are presented in Fig. 8 and Tables S3  and S4.The findings indicate that the degradation efficiency increased to an optimum value with increasing dye concentration from 5 to 20 ppm for crystal violet (97%) and from 5 to 15 ppm for methylene blue (93%).However, the rate of degradation decreased with a further increase in dye concentration, as this inhibited the   www.nature.com/scientificreports/electron transfer process on the nanoparticle surface between NaBH 4 and the dye, thereby slowing down and reducing the number of active sites on the catalyst 20 .

Effect of NaBH 4 concentration
The concentration of NaBH 4 was varied to determine the optimal concentration for degrading crystal violet and methylene blue while maintaining a fixed nanoparticle dose and dye concentration.The results revealed an increase in degradation with increasing NaBH 4 concentration from 0.01 to 0.12 M, reaching a maximum efficiency at 0.12 M (Fig. 9, additional 'Tables S9 and S10').Hence, 0.12 M was identified as the critical concentration, highlighting the necessity of an appropriate BH 4 −1 ion concentration for optimal nanoparticle degradation.Moreover, the higher NaBH 4 concentration elevated the local electron density on the FeNP surface, potentially leading to accelerated reaction rates 56 .

Effect of reaction time
The response over time (Fig. 10, see Tables S5 and S6) revealed that the degradation efficiency of both dyes increased with reaction time, peaking at 10 min for crystal violet (95.70% degradation) and 15 min for methylene blue (95.15% degradation).Beyond these durations, the efficiency plateau due to the active sites on the photocatalyst surface may become saturated or deactivated over time, suggesting that extended reaction times did not yield further enhancements in degradation efficiency 57 .www.nature.com/scientificreports/

Effect of pH
The pH contributes to determining the acidity or basicity of a solution and is an important parameter affecting catalytic degradation.This study explored the optimization of pH levels within a range from 2 to 12 using initial concentrations of 20 ppm crystal violet, 15 ppm methylene blue, and 0.025 and 0.075 g of FeNPs.The results (Fig. 11, see Tables S7 and S8) revealed that the highest degree of crystal violet degradation (97.47%) occurred at pH 4, while the highest degree of methylene blue degradation (94.22%) occurred at pH 5. Subsequently, increasing the pH to 12 resulted in a gradual decrease in degradation percentages for both dyes (efficiency increases in acidic medium and decreases in basic medium), potentially due to the formation of ferrous hydroxide, which could have occupied active sites on the FeNPs, subsequently reducing their reduction capacity 58 .

Catalytic reduction and degradation of CV and MB
The reduction of dyes by FeNPs can be explained through the electron transfer effect.Iron, a good conductor, can facilitate the transfer of electrons between donors and receptors.Thus, this catalytic process is mediated by iron nanoparticles via a redox mechanism, allowing for the transfer of electrons from the donor (BH 4 − ) to the acceptors (CV and MB).As depicted in Fig. 12a, the reduction of dyes in the presence of FeNPs is initiated by NaBH 4 .Subsequently, with the assistance of FeNPs as a catalyst, electrons are transferred from BH 4 − to CV and MB, leading to the reduction of CV and MB into colorless and nontoxic LCV (leucocrystal violet) and LMB (leucomethyene blue), respectively, by removing chromophore groups such as azo, nitro, N-C and S-C conjugated systems, as indicated in Fig. 12b (see F1-SI for the detailed mechanism).The reduction of CV and MB by the FeNP catalyst in the presence of NaBH 4 can be elucidated using the Langmuir-Hinshelwood model 59 .According to this model, the initially adsorbed BH 4 − ions donate electrons to the FeNPs (as shown in Eq. 1).As a result, a negatively charged layer develops around the FeNPs 60 .The reduction reaction at the FeNP surface is followed by the transfer of electrons from the FeNPs to cationic dye molecules through electrostatic interactions 59 .Previous work on PNP reduction by Ag/SiO 2 NC catalysts has also shown that these catalysts adhere to the Langmuir-Hinshelwood mechanism 61 .This model involves both molecules adsorbing onto the catalyst surface before engaging in the bimolecular reaction.When NaBH 4 is in excess, electrons and hydrogen are transferred to the surface of the nanocatalyst, where they control the rate of the reaction through the adsorption of dyes and borohydride ions (BH 4 − ) on the nanocatalyst surface.
The present study investigated the efficacy of an FeNP catalyst in the presence of NaBH 4 as a reducing agent for the degradation of MB and CV dyes, which are commonly found in industrial effluents.The reduction process was found to be highly efficient, with removal efficiencies of 94.22% for MB and 97.47% for CV within 10 and 20 min, respectively, under optimized experimental conditions (Fig. 11).NaBH 4 was preferred as the reducing agent due to its excellent electron-donating ability, facilitated by BH 4 − ions.The cationic nature of the CV and MB dyes facilitated their strong attraction toward FeNPs, which generated a negative layer that facilitated quick electron transfer to the dye molecules.NaBH 4 also serves as a source of hydrogen, which targets organic dyes after electron transfer to FeNPs.The resulting reaction produced reduced, colorless forms of methyl blue and crystal violet 27 .Subsequently, LCV and LMB desorbed spontaneously from the surface of the FeNPs and diffused into the solution due to weaker electrostatic interactions between the FeNPs and the colorless degradation products 62 . (1)

Kinetics studies
In this study, various kinetic models were used to analyze the experimental data and determine the degradation mechanism of dyes.Specifically, three kinetic models were established: the pseudo zero-order, pseudo first-order, and pseudo second-order models.The degradation reactions were conducted under optimal experimental conditions at different time intervals ranging from 5 to 20 min while keeping all the other parameters constant.To investigate the pseudo kinetics, calibration curves were constructed, as depicted in Fig. 13.These curves were used to calculate the concentration of the dyes after degradation at different time intervals.The data for the calibration curves of crystal violet and methyl blue are presented in Table 'S13' .
Pseudo kinetic experiments were also conducted, and the data used for calculating the residual concentration of the dyes are presented in the table in ''Table S14'' .From Table S14, the final concentrations of both dyes were calculated using their respective calibration curves, and other quantities used for investigating the pseudo kinetics were calculated and are shown in Table 1.were generated for the pseudo zero-order, pseudo-first-order, and pseudo-second-order kinetics models, respectively.Linear graphs with correlation coefficients for all the kinetics models are presented in Fig. 14a,b, and  c.The correlation coefficients and rate constants for each kinetic model were calculated and are summarized in Table 2.According to the results of the analysis of pseudo zero-order kinetics, compared with the other models, the FeNPs exhibited high correlation coefficients close to unity (R 2 = 0.999 and R 2 = 0.995) for the degradation of methylene blue and crystal violet, respectively.These findings suggest that the degradation process best-fitting a pseudo zero-order kinetics model.Specifically, a pseudo zero-order reaction was observed when the surface of the catalyst reached saturation, and the degradation rate became independent of the dye concentration and remained constant over time.The results presented in Table 2 for pseudo-first-order kinetics reveal a relatively high correlation coefficient for only the catalytic degradation of methylene blue.These findings suggest that the catalytic degradation process occurring on the surface of the FeNP catalyst follows pseudo-first-order kinetics, where the rate of catalytic degradation and concentration of dye are directly proportional.Compared to those of other pseudo kinetic models, such as pseudo zero-order and pseudo-first-order models, the correlation coefficient of the pseudo-second-order model (as shown in Fig. 14c) is lower for both dyes.This indicates that the catalytic degradation of both dyes does not fit the pseudo-second-order model.In general, the high correlation coefficient suggested that the zero-order pseudo kinetic model is a suitable model for describing the degradation kinetics of both dyes by FeNPs.This finding is important because it indicates that FeNPs can be used as effective catalysts for the degradation of these dyes in wastewater.

Study of the reusability of iron nanoparticles
Currently, the reusability of catalysts is a significant issue because a large number of catalysts are deactivated after the first or second cycle and are subsequently discarded.The reusability and stability of catalysts are vital for determining their performance, as an active and stable catalyst could considerably decrease the cost of the process.Here, crystal violet and methylene blue were used to study the reusability of FeNPs under optimum experimental conditions.The reusability of the FeNPs was tested four times.Figure 15 and ''Tables S11 and S12'' show that the reusability of the FeNPs was good.The findings of this study indicate that the FeNPs synthesized through green methods were relatively stable and could serve as effective catalysts, with only a slight reduction in their degradation rate after being used four times.These results are consistent with those of a previous study in which amorphous FeNPs synthesized from an aqueous extract of Boswellia serrata, a renewable natural resource, were utilized.In that study, the FeNPs were found to retain their catalytic activity even after being recycled up to five times, with only a modest decrease in their effectiveness 63 .Thus, the results of both studies suggest that green synthesized FeNPs have the potential to be used as efficient and sustainable catalysts in various applications.

Chemicals and reagents
The following analytical grade reagents were used directly as obtained without any further treatment throughout the work: ferric chloride, FeCl 3 (99.99%,Sigma-Aldrich, Indian) were used for the synthesis of iron nanoparticles as metal ion precursors.Crystal violet (AR, Samir Tech-Chem, Ltd., India) and methylene blue (AR, UniChem Ltd., India) dyes were used for the evaluation of the catalytic activity of the prepared iron and nanoparticles, and sodium borohydride, NaBH 4 (97% extra pure Merck, India), ethanol, CH 3 OH (97% extra pure Sigma-Aldrich, India), sodium hydroxide, NaOH (99.99%,Sigma-Aldrich, India) and hydrochloric acid, HCl (37%, Sigma-Aldrich, India) were used for adjusting the pH of the sample.Deionized water was used throughout the experiment to prepare the solutions, plant extracts and wash the plants.

Instrument and apparatus
A digital electronic balance (Model JA103P, China) with a 160 gm loading capacity was used to measure the leaf sample and all the other chemicals.A magnetic stirrer hot plate (UK) was used for stirring and maintaining the required temperature of the metal ion precursor and plant extract during the preparation of the iron nanoparticles.The extent of degradation of the dyes was monitored using a UV-Vis spectrophotometer (CECIL CE1021, USA).After degradation, the dispersed iron nanoparticles were separated from the treated solution using an 80-2 centrifuge at a maximum speed of 5000 rpm.Fourier transform infrared spectroscopy (65 FT-IR Perkin Elmer Spectrum, USA), an ultraviolet-visible spectrophotometer (SM-1600 spectrophotometer, USA), a powder X-ray diffractometer (Shimadzu XRD-7000S, Japan), and scanning electron microscopy (JEOL/EO-JCM-6000 plus, Japan) were used for characterization of the synthesized iron nanoparticles.www.nature.com/scientificreports/

Preparation of Vernonia amygdalina leaf extract
The collection of plant material for this study was initiated after obtaining the necessary permit from the EWCA Hawassa town office, with permit number EWCA-H-TT0092/23.In accordance with the World Health Organization (WHO) Quality Control Methods for Medicinal Plant Materials collection and preparation procedures 64 , mature and healthy Vernonia amygdalina leaves were collected from Hawassa Tabor Mountain Wild Life Conservation and Repair Park in the Sidama Regional State, Ethiopia, which is approximately 251 km away from Addis Ababa.The plant was authenticated by Botanist Reta Regassa, and a voucher specimen with the number TT00129/23 was deposited at the HU Herbarium, Hawassa University, for future reference.The collected leaves were cleaned thoroughly using running tap water to eliminate debris and contaminants, followed by deionized water and air drying at room temperature for one week.The aqueous extract of the plant was prepared by grinding the leaves with a mortar and pestle and boiling them in deionized water at approximately 60 °C until the color of the aqueous solution changed to brown-red (Fig. 16).The extract was cooled to room temperature and filtered using Whatman No. 1 filter paper.Finally, the extract was stored in a refrigerator at 4 °C for further experiments.

Biosynthesis of iron nanoparticles (FeNPs)
Iron nanoparticles were synthesized as described previously 65 with slight modifications (as shown in Fig. 17).
Ferric chloride hexahydrate (FeCl 3 •6H 2 O) was used as the metal precursor.Then, 75 mL of the Vernonia amygdalina leaf extract was added dropwise to 75 mL of 0.1 M FeCl 3 •6H 2 O solution at a 1:1 ratio at room temperature.The resultant mixture was stirred using a magnetic stirrer for 5 min, and the formation of a grayish black solution confirmed the synthesis of the iron nanoparticles.The nanoparticles were separated by centrifugation at 5000 rpm for 15 min and cleaned by subsequent washing with ethanol and water 3 times.The synthesized iron nanoparticles were finally dried at room temperature and stored in a sealed tight container for further use.

Characterization of synthesized iron nanoparticles
The synthesized iron nanoparticles were characterized using Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, powder X-ray diffraction (XRD) and scanning electron microscopy.

Catalytic degradation performance of iron nanoparticles
The catalytic activity of the synthesized iron nanoparticles was studied by degrading crystal violet and methylene blue in the presence of sodium borohydride as a reducing agent.To evaluate the effectiveness of the synthesized iron nanoparticles, an optimized amount of the synthesized nanoparticles was added following the addition of sodium borohydride to a beaker containing 25 mL of the prepared crystal violet and methylene blue aqueous solutions.A beaker containing a mixture of dye solution, sodium borohydride and nanoparticles was stirred on a magnetic stirrer.The mixture was subsequently centrifuged at 1500 rpm for 5 min to separate the nanoparticles from the degraded dye solution by preventing nanoparticle dispersion.The extent of degradation was monitored by measuring the absorbance before and after degradation at 590 and 664 nm for crystal violet and methylene, respectively.

Catalyst dose
For the optimization of the catalyst dose, catalyst doses ranging from 0.010 gm to 0.125 gm with a range of 0.025 gm were used per 25 mL of 10 ppm crystal violet and methylene blue at a constant value of the other parameter.Then, the catalytic degradation efficiency was calculated.

Initial concentration of dye
To investigate the effect of the initial concentration of dyes on the catalytic degradation efficiency, different initial concentrations of both dyes were tested (5, 10, 15, 20, 25 and 30 ppm for crystal violet and methylene blue).The catalyst dose was fixed at 0.025 gm/25 mL for crystal violet and 0.075 gm/25 mL for methylene blue.Prior to conducting these experiments, other parameters were optimized and applied while remaining constant.

Reaction time
To investigate the total reaction time required for the complete degradation of crystal violet and methylene blue, six trials were conducted with 5 min intervals between each trial.The catalyst dose was fixed at 0.025 gm/mL for crystal violet and 0.075 gm/25 mL for methylene blue, while the initial concentrations of crystal violet and methylene blue were fixed at 20 ppm and 15 ppm, respectively.Prior to conducting these experiments, other parameters were optimized and applied.

Concentration of sodium borohydride
The effects of the concentration of the reducing agent (NaBH 4 ) on the degradation of crystal violet and methylene blue were investigated.To determine the optimum concentration of reducing agent for the best catalytic degradation, various concentrations of reducing agent were tested from 0.01 to 0.20 M. The catalyst dose was fixed at 0.025 gm/25 mL for crystal violet and 0.075 gm/25 mL for methylene blue, while the initial concentrations of crystal violet and methylene blue were fixed at 20 ppm and 15 ppm, respectively.Prior to conducting these experiments, other parameters were optimized and applied.

pH of dyes
To study the effects of pH on the degradation of both dyes, all the other parameters were kept constant, the pH was varied from pH 2 to pH 12, and the optimum pH was investigated.

Determination of degradation percentage
The degradation percentage of the dye solution was calculated by taking the absorbance values recorded at λ max values of 590 nm and 664 nm for crystal violet and methylene blue, respectively, before and after catalytic degradation.Equation ( 2) is provided as follows: where A o = the initial absorbance of the dye solution before exposure to sunlight and A t = the absorbance of the dye solution at time (t). (

Kinetics study
In this investigation, all the measurements were performed under optimized experimental conditions.Calibration curves were constructed for both dyes to calculate the concentrations of the dyes after degradation at different time intervals.Then, a constant concentration (initial concentration with high degradation efficiency) was used for both dyes, and the extent of degradation was studied within 5 min intervals.The concentration of dye was subsequently calculated by applying the Beer-Lambert law, as shown in Eq. ( 3): where A-Absorbance, ∈-Molar absorptivity constant, b-Path length, C-Concentration.This equation is related to y = mX + b, and from the absorbance versus concentration data, the slope (m) was calculated and used to calculate the concentration after degradation.Finally, pseudozero-order, pseudofirst-order and pseudo-second-order kinetics were investigated according to Eqs. 4, 5 and 6, respectively, as mentioned below.

Instrumental calibration
Standard stock solutions of dyes were taken for calibration of the instrument for dyes used for the experiment.For instrument calibration, first, 1000 ppm stock solution of each dye was prepared, and an intermediate standard solution containing 100 ppm was prepared in a 500 mL volumetric flask.The intermediate standards were subsequently diluted with deionized water to obtain six working standards for each dye of interest for calibration purposes.

Study on catalyst reusability
The reusability of the catalyst was investigated during the degradation process under identical experimental conditions (at the optimum concentration of dye solution, reducing agent, catalyst dose, and time).After the completion of the reaction, the catalyst was separated by centrifugation from the reaction mixture, washed with ethanol and water, dried and reused for four consecutive cycles.

Conclusion
Iron nanoparticles (FeNPs) were successfully synthesized by a green method using Vernonia amygdalina plant leaf extract as a natural reducing and capping agent.The process is relatively easy, fast, cheap, and environmentally friendly and does not require any organic solvents or other toxic reagents.Biosynthesized FeNPs were characterized with different analytical techniques, such as UV-visible, FT-IR, XRD, and SEM, and the obtained instrumental analysis data revealed continuous absorption at the SPR in the visible range, suggesting the formation of amorphous FeNPs, the presence of various functional groups, indistinct diffraction peaks that reveal predominantly amorphous FeNPs and irregular morphology with a lack of uniform distribution in shape and size and an average particle size distribution of approximately 2.31 µm.The biosynthesized FeNPs were also applied for the catalytic degradation of MB and CV in the presence of NaBH4 and showed maximum catalytic degradation efficiencies of 94.22% and 97.47%, respectively, under optimum conditions for each dye.The kinetics study exhibited high correlation coefficients close to unity (0.999 and 0.995) for the degradation of MB and for the degradation of CV, respectively, for the zero-order pseudo kinetics model, which indicates that the model is highly suitable for the degradation of both dyes by FeNPs compared to other models.The reusability and stability of the biosynthesized nanocatalysts were studied, and the catalysts were successfully used as efficient catalysts, with a slight decrease in the degradation rate of more than four times.The results from this study showed that the use of biosynthesized FeNPs is a cost-effective, environmentally friendly, and efficient means for the highly efficient catalytic degradation of toxic pollutant dyes discharged from industries such as medicine, cosmetics, paints, plastics, and textiles.

Figure 1 .
Figure 1.Visual observation of the green synthesis of FeNPs.

Figure 5 .Figure 6 .
Figure 5. SEM images (a and b); Histogram of PSA (c and d) in the biosynthesized FeNPs.

Figure 7 .
Figure 7. Effect of catalyst loading on the degradation of CV and MB.

Figure 8 .
Figure 8.Effect of initial concentration on the catalytic degradation of CV and MB.

Figure 9 .
Figure 9.Effect of sodium borohydride concentration on the catalytic degradation of CV and MB.

Figure 10 .
Figure 10.Effect of reaction time on the catalytic degradation of CV and MB.

Figure 11 .
Figure 11.Effect of pH on the catalytic degradation of CV and MB under optimized conditions for all the other parameters.

Figure 12 .
Figure 12.Schematic diagram of the catalytic reduction reaction (a) and mechanism of the catalytic degradation of CV and MB by the NaBH 4 and FeNP samples (b).

Figure 13 .
Figure 13.Calibration curves of violet and methylene blue.

Figure 14 .
Figure 14.Pseudo zero-order (a), pseudo-first-order (b) and pseudo-second-order kinetic models for the catalytic degradation of CV and MB by FeNPs with NaBH 4 .

Figure 17 .
Figure 17.Flow diagram for the biosynthesis of FeNPs.

Table 1 .
Data for examining the pseudo kinetics of heterogeneous catalysis by FeNPs.

Table 2 .
Rate constant (k) and correlation coefficient (R 2 ) calculated for the kinetic model.